It is known, by the present assignee's own work, how to form and print microscopic vertical light emitting diodes (LEDs), with the proper orientation, on a conductive substrate and connect the LEDs in parallel to form a light sheet. Details of such printing of LEDs can be found in U.S. Pat. No. 8,852,467, entitled, Method of Manufacturing a Printable Composition of Liquid or Gel Suspension of Diodes, assigned to the present assignee and incorporated herein by reference.
FIG. 1 is a cross-sectional view of a single pre-formed LED 16 that is mixed with other LEDs in a printable solution, to form an LED ink, and then printed on any surface in any pattern. Each LED 16 may have a width on the order of a human hair and may have a diameter between 10-200 microns. There are many ways to print the LEDs 16, such as screen printing, gravure printing, flexography, etc. The printing process may print the LED ink in any pattern, such as a two-dimensional shape, but the individual LEDs will be randomly located on the substrate surface as a natural result of the printing process. In the example of a blue-emitting LED 16, each LED 16 includes semiconductor GaN layers 17, including an n-layer, an active layer, and a p-layer. LEDs emitting other wavelengths may use other materials. A phosphor may be used to color-convert the light.
An LED wafer, containing many thousands of vertical LEDs, is fabricated so that the bottom metal cathode electrode 18 for each LED 16 includes a reflective layer. The top metal anode electrode 20 for each LED 16 is relatively narrow to allow almost all the LED light 21 to escape the anode side. A carrier wafer, bonded to the “top” surface of the LED wafer by an adhesive layer, may be used to gain access to both sides of the LED for metallization. The LEDs 16 are then singulated, such as by etching trenches around each LED down to the adhesive layer and dissolving the exposed adhesive layer or by thinning the carrier wafer.
The microscopic LEDs are then uniformly infused in a solvent, including a viscosity-modifying polymer resin, to form an LED ink for printing.
If it is desired for the anode electrodes 20 to be oriented in a direction opposite to the substrate after printing, the electrodes 20 are made tall so that the LEDs 16 are rotated in the solvent, by fluid pressure, as they settle on the substrate surface. The LEDs 16 rotate to an orientation of least resistance. Over 90% like orientation has been achieved.
Once the LEDs 16 are printed as a monolayer on the substrate, they are in a non-deterministic random array as a natural result of the LEDs being randomly located within the solvent when printed.
In some applications, it may be desirable to precisely position an individual lens, a phosphor, quantum dots, or other optical structure directly over each printed LED 16 without covering areas between the LEDs 16. However, this is impractical using prior art techniques since the LEDs 16 are very small and randomly located.
What is needed is a technique for precisely positioning optical structures directly over printed LEDs, where the technique is suitable for a high speed manufacturing process, such as a roll-to-roll manufacturing process.